With the increasing threat of countermeasures and detection of active (e.g., laser) rangefinders and the emphasis on stealth, a requirement exists for passive rangefinders.
Prior art military devices have typically used laser rangefinders to determine a range. Laser rangefinders have the disadvantage that they are active devices. Their emitted radiation may be detected. This detection may not only give away the soldier's position, it may also result in countermeasures, including return fire.
There are also a number of parallax systems used to determine range where two images are formed at fixed opposing angles and the length of a baseline is adjusted to determine range. Still other systems use the blur of the image to determine the offset from the focal point to estimate the range. None of these techniques are entirely satisfactory.
Accordingly, what is needed in the art is a better rangefinder and a better way to determine a range to a target point, namely a passive rangefinder or passive way to determine a range to a target point.